Electrical network



July 15,1924.

K. S. JOHNSON ELECTRICAL NETWORK Filed 515:!) ll 1921 2 sh eets sheel 1 //7Ve/7/0/.' fi8/7/76/fi 5. Job/750M. 4v M K. S. JOHNSON ELECTRICAL NETWORK Filed May 11. 1921 2 Sheqts-Sheet 2 Combined Aftenualion of Three Sehfions Frequency lm/emor: Kenna/h 5. J hnson by WZZMW flflj Patented July 15, 1924.

UNITED STATES PATENT OFFICE.

KENNETH S. JOHNSON, OF JERSEY CITY, NEW JERSEY, ASSIGNOR TO' WESTERN ELEG- TEIC COMPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

ELECTRICAL NETWORK.

Application filed May 11,

To all whom it may concern:

Be it known that I, KENNETH 8. JOHN- SON, a citizen of the United States, residing at Jersey City, in the county of Hudson, State of New Jersey, have invented certain new and useful Improvements in Electrical Networks, of which the following is a full, clear, concise, and exact description.

This invention relates to electrical circuits for the transmission of alternating currents and more particularly to wave filters designed to permit the transmission of certain frequencies and suppress other frequencies.

It has been found that-'a network in the form of a Wheatstone bridge, having certain impedances in its arms and having the incoming and outgoing lines connected respectively across its pairs of diagonal terminals, forms a balanced filter section. Filters having bridge type sections may be made to have the electrical characteristics of the filters shown in U. S. patent to Campbell, 1,227,113, dated May 22, 1917, and 1n the -5 application of G. A. Campbell, Serial No.

239,576, filed June 14, 1918, as well as other similar filters not shown by Campbell.

A great number of combinations of inductances and capacities are possible in a bridge network having filter properties. A few of these are shown herein, and design formulae are developed for some of them. General formulae are given from which specific formulae not given herein, may be derived by obvious processes, and general rules are given to enable one to determine what conditions are necessary in order that a given network may have filter properties.

In the accompanying drawings, two of the bridge arms are crossed in order that the incoming and outgoing line terminals may appear at the respective sides of each figure.

In the drawings, Figs. 1 to 6 represent respectively, filter sections in which each arm has two reactances; Figs. 7 to 16 represent sections in which two or all of the arms have each only one reactance; Fig. 17 shows diagrammatically a general case of bridge section; Fig. 18 shows a T network which is the electrical equivalent of that shown in 1921. Serial No. 468,710.

Fig. 17; Fig. 19 is a diagrammatic illustration of a filter comprising two bridge type sections and one series shunt type section; and Fig. 20 is a graphic illustration of the attenuation characteristic of the filter shown in Fig. 19.

Fig. 17 shows a section of a general brldge type filter, in which two of the arms have equal impedances A in order that the network may be equivalent to a symmetrical T network, as will be explained below, and the other two arms have impedances B and C whlch may be unequal. In the following theoretical consideration it is assumed that there is no resistance in the arms. It is found in practice that this assumption is sufliciently justified for design purposes. It 15 found more convenient to reduce the network of Fig. 17 to the equivalent T network of Fig. 18, having equal series impedances w, a: and a shunt impedance y. The T network is externally equivalent to the bridge network when w=A (1) and y: BC-A (A+B) (A+C) This is shown by setting the short-circuited and open-circuited impedances of'the two circuits equal to'each other. Equating the open circuited impedances of Figs. 17 and 18,

52 My (A+O)+( (3) Equating the short-circuited impedances,

.. my AC AB (4) by Campbell) lies between and 4, and Equations for these limiting values for practical y extinguishing all. other frequen- Figs. 17 and 18 corresponding to Campbells cies, equations (4) may be written as follows 2- z, 22: 2A 2A(2A+B+O) 0 g BO-A Bc-A (A B) (A O) d i or A may be series resonant and B parallel resonant or vice-verse. W1: .4 (6) I It follows also that to have filter action in a circuit having inductance only in one It has already been assumed that the series P l 0f fi p y y 111 the o her impedances A of each section are equal. Let P 111 mg the values of the reactus now consider the case where the shunt mots 111 arms f ne palr must be unimpedances B and C are equal. Assuming q that B must be unequal t0 t B C equations (5) d (6) may b may be noted, however, that the circuit Written shown in Fig. 15, when the incluctances are A B equal and the capac1t1es are equal, has ya- M 0 (7) none desirable characteristlcs, some of which are described in the application of George A A. Campbell, Serial No. 455,670, filed March 1 (8) 25, 1921.

From. equations (7) and (82,- it is seen By inverting both sides and simplifying, that W119? l' g yp fi tars P355311 equation (7) reduces to frequencies for which the value of B m A 2r (9) TA lie between zero and 1. The value of this expression lies between these limits when A and B are of opposite sign. It follows that frequencies will be passed for which A and B B are of opposite sign and that frequencies 0 will be suppressed for which A and B are of the same sign. a condition which can exist only when B=O 011 the other h y a g B and G and A is finite, or when A: c0 and B i unequal, all bridge type circuits, except fimt It f ll th t if 13:0 d it i 1 those having only one kind of reactance, are sired to transmit only a band of frequencies both limits of which are other than 0 GT8), Letting 6 represent the p qp g n A and B each must compriseatuned circuit, Stant 0f h g a filter fiectwn 0f gboth being series resonant or both being and substituting in equation (2) of Patent arallel resonant, In Figs, 1 and 6 are NO. 1,227,113, above referred t0, the value Of shown circuits in which this is possible. If,

It is evident from inspection that equation (9) can be true only when A=O and B is finite, or when B= w and A is finite. By inversion, equation reduces to however, one of the limits of each band transmitted can be 0 or as in a high pass, Z,

low pass, or band elimination filter, it is suflicient for A or B to be a resonant circuit, from equation (5) above,

For the case above referred to, where pedance Z as indicated at the right in Fig. B=C, equation (11) reduces to- 18. Then the impedance Z measured from the left-hand terminals is as followsz- 1 B+A 0=O0Sl1 m ZL) I BC-A ZL=A+ Assume the network of Fig. 17 or Fig. 18 A Z BG-A to be terminated in its own iterative 1m- (A+ E) (5 +U) Solving for Z B(A+C)+C(A+B) (A B) (A C) If B=C, equation (14) reduces to and Substituting these values in equation (9),

The condition represented by equation (18) is true when w=0,orf==0 19 Similarly, from equation (10) we have Solving for a),

1 1 21 va W J in which f is the cut-off frequency.

It is seen from equations (19) and (21) that the network of Fig. 8 is a low pass filter, passing all frequencies from 0 to 1 21TV L 0 The iterative impedance of the filter of Fig. 8, from equations (15), (16) and (17 1s If the impedance at zero frequency be represented by Z equation (22) becomes In designing a low pass filter, it is usually desirable to make the value of Z equal to the impedance of the circuit to which the filter is to be connected. There is an advantage therefore in writing the equation for the iterative impedance as follows:

Equation (24) is obtained from equation (22) by substituting therein the value 1 L C =m from equation (21) and the value from equation (23).

en B:C, as assumed in Fig. 8, equation (2) becomes When y=0, that is when BzA, no current can go through the filter section. Letting f be the frequency at which this condition of maximum attenuation exists, and w =21rf and equating the values of B and A from equations (16) and (17 foo ; 1 WWELAOFOZ) Letting the ratio of f w to f be represented by a, we have from equations (21) It will be observed that by making C small as compared with C the value of 0;

can be made to approach unity as closely as desired, thus giving the filter section of Fig. 8 as sharp a cut ofl:' as desired. The electrical characteristics of this form of bridge filter are the same as those of the filters shown in Figs. 9 and 10 of Gampell application, Serial No. 239,576, above referred to.

By substituting the values at A. and B from equations (16) and (17) m equation (12), we have, as the propagation constant of Fig. 8, 1i 0 w l-L O,w 7 0 w 1L,0 w

6: cosh- Assume by way of illustration that it is desired to design a low pass filter to work between impedances of 600 ohms, to have a cut off frequency of 3000 cycles, and to have infinite attenuation at 3300 cycles, or 1.10 times the cut off frequency. Substituting the values Z GOO, f -3000, and (121.10, in equations (30) (31) and 32, we obtain directly C :.O3682 microfarads.

L,:.O1325 henries.

O, .2124 microfarads.

Fig. 9 differs from Fig. 8 in that the 1nductances L',, L,, are wound on the same core. The filter characteristics of the two circuits are the same. It is true in general that, where impedances in, both arms of a pair of opposite arms are equal, the corresponding inductances can be wound on the same core. For example, in Figs. 1 and 3, the inductances L can be wound on the same core and the inductances L (assumed to be equal) can be wound on the same core.

By methods similar to the above, design formulae can be derived for each of the other forms shown. For example, Fig. 13 represents a band pass filter having cut oif frequencies- Fig. 15 represents a band elimination filter having cut off frequencies-- In the drawings, in those forms in whichv B may be e ual to C, the reference characters L an C are used to represent the reactances in both of the crossed arms. It is to be understood, however, in these cases that the reactance or reactances in one crossed arm may be different from the corresponding reactance or reactances in the other arm. In those forms in which B can not be equal to C, the reference characters L L and C,,, C, are used for the reactances in the respective crossed arms.

For reference purposes, Figs. 1 to 16 are listed below and are each classified as being a high pass, low pass, band pass, or band elimination filter. It is believed that these terms are self-defining.

Fig. 1, double band pass, becoming single band pass, when BzC.

Fig. 2, double band pass, becoming single band pass, when B resonates at the same frequency as C.

Fig. 3, double band elimination, becoming single band elimination, when BIC.

Fig. 4, double band elimination, becoming single band elimination, when B: C.

Fig. 5, double band pass, becoming single band pass, when B resonates at the same frequency as C.

Fig. 6, double band Sass, becoming single band pass, when B:

Fig. 7, double band pass, with one band extending to infinity, becoming high pass, when BIC.

Fig. 8, double band pass, with one band eBxteEding to zero, becoming low pass, when Fig. 9, double band pass, with one band eBxteIding to zero, becoming low pass, when Fig. 10, double band pass, with one band ilaztegding to zero, becoming low Fig. 11, band pass.

Fig. 12, double band pass, with one band extending to infinity, becoming high pass, when 13:0.

Fig. 13, band pass.

Fig. 14, double band pass, with one band extending to infinity, becoming high pass, when BzC.

Fig. 15, band elimination.

Fig. 16, double band pass, with one band extending to zero, becoming low pass, when B::C.

It will be noted that when 3:0, Figs.

and

pass, when 3, 7, and 8 are identical with Figs. 4, 12, and 16. respectively. p

While each figure of the drawing shows only a single filter section, it is to be understood that the invention contemplates connecting as many sections in tandem as may be necessary to obtain the desired amount of suppression in the attenuated range. The sections may be identical, but for best results it is preferable to employ sections having the same cut off frequency or frequencies and substantially the same iterative impedance, but having different frequencies of maximum attenuation or suppression. This can easily be done with a filter having three separately variable reactances, such as in Fig. 8. From equations (21) (23) (24) and (28) it will be noted that the cut off" fr uency 7, is a function of the product L, (1,, the iterative impedance Z is a function of and frequency, and the ratio of the fre- .quency of maximum suppression f on to f is a function of It is possible, therefore, while keeping f, and Z substantially constant to vary from avalue closely a preaching one to a value approaching in ity. Furthermore, good results can be obtained by comb nmg a section corresponding to Fig. 8, with a section of the two element filter shown in Fig. 7 of Campbell Patent No. 1,227 ,113, in WhlOh f is inherently infinity.

In Fig. 19 is shown a filter having two sections of the form shown in Fig. 8 and one section of the series shunt type shown in Campbell Patent 1,227,113, Fig. 7. Although the third section is of the same form: as the first section, its constants are changed so as to give maximum attenuation at a frequency different from that of the first section. The cut-off frequency must be the same. The values of L C C and L C and C of the first and third sections are determined by the formulae given above. It is apparent from the formula I of the Campbell patent that the cut-off frequency of the second section is equal to By methods similar to those used above in deriving formula (2%), it can readily be shown that the characteristic impedance of the second section is equal to It" will be seen therefore that a sufiicient number of variables are present so that'the the range to be suppressed is illustrated in Fig. 20. i

What is claimed is: 1. A wave filter comprising a section havmg termmals for receiving electric waves, parallel reactive paths between said terminals, output terminals on said paths at points the. potentials of which are such that waves of a band of frequencies are substantially' attenuated-and waves of an adjacent band of frequencies are passed substantially without attenuation, said output terminals having substantially equal potentials for a frequency in the attenuated range relatively near the cut-off frequency whereby Waves of said frequency are substantially wholly suppressed, and a filter section connected in tandem therewith having the. same cutoff frequency but having a different frequency of maximum suppression.

2. A Wave filter comprisin r a section in the form of a VVhe-atstone bridge having reactances in its arms, and havinginput and output terminals at its diagonal terminals, the values of said reactances being such that waves of a band of frequencies are substantially attenuated and waves of an adjacent band of frequencies are passed substantially without attenuation, and a section connected in tandem with said first section, said first and second sections having the same ranges of passed and attenuated frequencies, but having different attenuation characteristics.

3. A wave filter comprising a section having terminals for receiving electric waves, parallel reactive paths between said terminals, output terminals on said paths at points the potentials of which are such that waves of a band of frequencies are substantially attenuated and waves of an adjacent band of frequencies are passed substantially without attenuation, and a second section connected in tandem with said first section, said second section havin impedances in series and in shunt respectively with reference to currents propagated through the filter, said first and second sections having the same ranges of passed and attenuate frequencies.

4. YA wave filter comprising a section. having terminals for receiving electric waves, parallel reactive paths between said term1- nals, output terminals on said paths at points the potentials of which are such that waves of a band of frequencies are subs-tam tially attenuated and'waves of an adjacent band of frequencies are passed substantially without attenuation, and a section connected in tandem with said first section, said first and second sections having the same ranges of passed and attenuated frequencies and having difiering attenuation characteristics. Y 5. A wave filter comprising a section havpoints the potentials of which are such that waves of a band of frequencies are substan: tiall attenuated and waves of an; adjacent ban of frequencies arepassed substantially without attenuation, and a section connected in tandem with said first section, said first and second sections having the same ranges of passed and attenuated frequencies and the same characteristic impedances but having; different attenuation characteristics.

11 witness whereof, I hereunto subscribe my name this 10th day of May A. D., 1921.

KENNETH s; JOHNSON. 

